Effect of molecular structure and hydrogen bonding on the fluorescence of hydroxy-substituted naphthalimides

Fluorescence properties of hydroxy-naphthalimides were studied in methylene chloride in the absence and the presence of hydrogen-bonding additives. The position of the HO-substituent only slightly affects the radiative rate, however, the triplet yield and the rate of the radiationless processes are considerably higher for the 3-hydroxy derivative. Addition of nitrogen-heterocyclic compounds leads not only to hydrogen-bonding in the ground state but also fluorescence quenching. The parallel change throughout the series of the hydrogen-bond acceptors between the proton affinity and the rate constants of dynamic quenching indicates that proton displacement plays a crucial role in the excited hydrogen-bonded complexes. Interaction of hydroxy-naphthalimides with pyridine and benzoxazole results in rapid radiationless deactivation from the singlet excited state, whereas intense emission as well as long fluorescence lifetime characterize imidazole and pyrazole complexes. The dual emission of the imidazole complexes observed in solvents of medium polarity is assigned to two conformers which differ in the extent of the proton shift along the hydrogen-bond.